In the production of oil, a well is drilled down to an oil bearing strata. At the bottom of the well, a motor/pump assembly is installed to pump the oil to the surface of the earth from the pool that gathers at the bottom of the well. FIG. 1 illustrates a typical oil well assembly. A well 10 is drilled into the earth perhaps thousands offeet down to an oil bearing strata. A motor 14 which drives a pump 16 are lowered to the bottom of the well. The motor 14 is electrically connected by a cable 22 to a drive system 20, typically located outside the well, which provides drive signals to the motor 14. The drive signals control the operation of the motor 14 which in turn controls the operations of the pump 16. When the motor 14 is turned on, the motor turns the pump 16 so that oil is drawn out of the bottom of the well and up the pipe 18 to the surface creating a positive flow 24.
One concern in the oil industry is the amount of time an oil well is not operating because of a physical or mechanical problem. The downtime of the pump reduces the production capability of the well. One source of downtime occurs when there is a power interruption to the system caused by a power outage, blown circuit breaker, controlled stop or the like. When a power interruption occurs, the drive system loses control of the motor because control signals can no longer be sent to the motor. Even though the motor is now unpowered, the motor and pump will continue to operate for at least a certain period of time depending on how fast the motor was turning at the time the power was interrupted. The speed of the motor will slowly decrease until the motor and pump come to a complete stop.
One problem unique to oil well applications and submersible pumps is that when the pump stops there is a column of oil, for example 4000 feet tall, resting on top of the pump. The column of oil will begin falling back to the bottom of the well due to gravity. As the oil falls back, the oil exerts pressure on the pump causing the pump to work in the opposite direction, i.e. a negative flow. In turn, the pump will cause the motor to rotate in an opposite direction and at varying speeds as the entire column of oil falls to the bottom of the well. As a result, when the power to the motor is interrupted, the motor will operate at different speeds and even in different directions depending on the length of the power interruption.
Since a motor can be damaged or a circuit will be tripped in the drive system if the motor receives an initial drive signal which is different from the actual speed and direction of operation of the motor, oil well operators have to wait until they are certain that the entire column of oil as fall back to the bottom of the well and the motor has come to a complete stop before they can send drive signals to restart the motor after the power interruption is over. This creates a tremendous amount of downtime regardless of how long the power interruption lasts.
Thus, there is a need for a control system which is capable of automatically restarting a motor of a submersible pump after a power interruption without having to wait for the motor to come to a complete stop.
Automatic restart programs are known for other applications, such as devices which include a driveable centrifuge. A centrifuge is used to separate solids in liquid samples by spinning the sample around a circle at high speeds. When the power is cut off to the motor in the centrifuge, the momentum and weight of the centrifuge will keep the samples spinning at decreasing speeds until the centrifuge comes to a complete stop due to friction and other forces. One known control system can restart the motor after the power is turned back on by first determining the actual speed of the unpowered motor and applying drive signals to the motor that match the actual speed of the motor. This is performed by analyzing the back EMF signals produced by the motor's residual magnetism created by the unpowered spinning motion of the motor. Once the speed of the motor has been determined, the matching drive signals are sent to the motor to regain control of the centrifuge and additional control signals can be applied to change the centrifuge's speed to a desired level. Thus, the operator does not have to wait for the centrifuge to come to a complete stop before restarting the motor after a power interruption.